Early cardiac pacemakers provided a fixed-rate stimulation pulse generator that could be reset, on demand, by sensed atrial and/or ventricular depolarizations. Modern pacemakers include complex stimulation pulse generators, sense amplifiers, and leads which can be configured or programmed to operate in single- or dual-chamber modes of operation, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit.
In recent years, single- and dual-chamber pacemakers have been developed which measure parameters which are directly or indirectly related to the patient's metabolic requirements (i.e., demand for oxygenated blood) and vary the pacing rate in response to such parameters. Such measured parameters include, for example, physical activity of the patient, right ventricular blood temperature, venous blood oxygen saturation, respiration, minute ventilation, and various pre-and post-systolic time intervals measured by impedance or pressure sensors within the right ventricle of the heart. Such sensor-driven pacemakers have been developed for the purpose of restoring rate response to exercise in patients lacking the ability to increase rate adequately by exertion.
In general, a rate-responsive pacemaker includes a sensor which produces an output that varies between a maximum sensor output level and a minimum sensor output level ("sensor output"). The pacemaker delivers pacing stimuli at a pacing rate ("pacing rate") which varies as a linear or monotonic function ("f") of the sensor output, between a selectable lower pacing rate ("lower rate limit") and upper pacing rate ("upper rate limit"). Function f has a selectable slope, where the slope of f corresponds to the ratio of pacing rate change to sensor output change. That is, the slope of f reflects the amount of change--increase or decrease--in the pacing rate resulting from an incremental change in sensor output. The slope of f is adjustable by means of an external programmer along with the lower and upper rate limit values. Thus, the pacing rate typically provided is equal to the programmed lower rate limit plus an increment which is a function of the measured sensor output, as follows: EQU pacing rate=lower rate+f(sensor output).
While this rate response technique provides a useful and workable system between programmed parameters, the behavior of the pacemaker is complex and often not readily apprehensible. Pacemakers that measure the physical activity of the patient by means of a piezoelectric transducer have become popular among rate-responsive pacemakers. Such a rate-responsive pacemaker employing a piezoelectric transducer is disclosed in U.S. Pat. No. 4,485,813 to Anderson et al. and assigned to the assignee of the present invention, which patent is incorporated herein by reference in its entirety.
Some temperature-sensing pacemakers have employed relatively more complex functions to take into account an initial dip in temperature due to the onset of exercise. One such pacemaker is described in U.S. Pat. No. 4,719,920 to Alt et al.
Furthermore, the decay slope of conventional activity-based rate-responsive pacemakers do not approximate the heart's normal behavior, in that they are programmed to follow a curve based on a single time constant. This discrepancy between the normal heart deceleration function at the end of physiologic stresses due to accumulated metabolic debt, and the conventional pacemaker decay function has not been rectified by any pacemaker presently available on the market and known to the inventors.
Thus, the inventors believe that it would be desirable to provide a cardiac pacemaker of the rate-responsive type which varies its attack and/or decay pacing rates in harmony with the heart's normal behavior.
In U.S. Pat. No. 5,134,997 filed Aug. 14, 1990, by Bennett et al. entitled "Rate Responsive Pacemaker and Pacing Method" (hereinafter referred to as the Bennett et al. reference) there is disclosed a rate-responsive pacemaker having a modified pacing rate decay curve after a period of increased activity. The method disclosed in the Bennett et al. reference includes the steps of selecting a set of predetermined achievement criteria such as an achievement rate and an achievement duration or time interval. The achievement rate is initially selected between an upper pacing rate and a first pacing rate switch threshold. The pacing method then determines whether the achievement criteria have been met. If the achievement criteria have been met, then the decay time constant of the pacing rate decay curve changes from a first value to a second value when the pacing rate drops below the first pacing rate switch threshold.
Further in accordance with the Bennett et al. reference, a second pacing rate switch threshold lower than the first pacing rate switch threshold is selected, and if the achievement criteria have been met, then the pacing rate decay time constant is modified from the second value to a third value when the pacing rate drops below the second pacing rate switch threshold. This third value of the pacing rate decay time constant may be equal to the first value.
According to Bennett et al., the decay rate time constant is not modified in the above-described manner if the achievement criteria have not first been satisfied.
The Bennett et al. pacemaker also periodically calculates a new activity pacing rate, and calculates a new activity target rate based upon the activity sensor output. The achievement rate is calculated as follows: EQU Achievement Rate=Lower Rate+A(Upper Rate-Lower Rate)
where "A" is a percentile value. Thus, if the programmed upper and lower rate settings define a range of possible pacing values, the achievement rate is defined as some percentage of that range. Similarly, the first pacing rate switch threshold is calculated as: EQU First Pacing Rate Switch Threshold=Lower Rate+U(Upper Rate-Lower Rate)
where "U" is a percentile value. The second pacing rate switch threshold is defined as: EQU Second Pacing Rate Switch Threshold=Lower Rate+10% of Lower Rate.
The Bennett et al. reference is hereby incorporated herein by reference in its entirety. As would be apparent to one of ordinary skill in the pacing art, the modified pacing rate decay curve disclosed by Bennett et al. comprises first and third decay phases defined by the programmed decay rate time constant, and a second decay phase, interposed between the first and third phases, in which the programmed decay rate time constant is temporarily replaced with a modified time constant. The boundary points between the first and second decay phases, and the second and third decay phases, are defined in terms of pacing rate. The transition from the first phase to the second phase occurs at a point where the pacing rate drops to the first pacing rate switch threshold, and the transition from the second phase to the third phase occurs at a point where the pacing rate drops to the second pacing rate switch threshold. Although the first and second pacing rate switch thresholds may be programmable values, the decay rate will only change at these programmed values. Moreover, while the programmed and modified time constants are programmable value, the pacing rate decay will occur at one or the other of these programmed values unless the decay parameters are re-programmed. Thus, the Bennett et al. pacemaker could be generally characterized as modifying decay curve by changing the time constant of the pacing rate decay, assuming the achievement criteria have been reached, during an interval determined by the current pacing rate.
By way of comparison, a pacemaker in accordance with one embodiment of the present invention can be generally characterized as modifying the decay curve by changing the time constant of the pacing rate decay, assuming the achievement criteria have been reached, during an interval determined not by the current pacing rate, but by a measure of the amount of work recently performed by the patient.
It is believed by the inventors that it would be advantageous to provide a pacemaker in which the decay time constant is modified based not strictly upon the current pacing rate, but also upon a measure of the patient's recent levels of exertion, the pacemaker will more effectively mimic the deceleration behavior of a healthy heart.